Polysilazanes with silicon-nitrogen-carbon compositions have been developed and their utility as precursors to silicon nitride-containing ceramic materials have been disclosed in U.S. Pat. Nos. 4,482,669; 3,853,567; 4,689,252; 4,612,383; 4,675,424; 4,722,988; 4,937,304.
In addition, some polysilazanes have been shown to be, under specific conditions, precursors to silicon carbide-containing ceramic materials (Matsumoto, R. L. K. Mat. Res. Soc. Symp. Proc., Vol. 180, p. 797).
European Patent Application 89 300563.7, L. M. Niebylski, discloses a preceramic polymer consisting of a silicon nitrogen backbone bonded with boron and oxygen used to coat carbon/carbon composites. In European Patent Application 89 300563.7, the silazane polymer is reacted with a boroxine.
U.S. Pat. No. 4,668,642, D. R. Bujalski, discloses a method to increase the ceramic yield of R.sub.3 SiNH-containing silazane polymers by mixing with boron compounds before pyrolysis. Suitable boron compounds disclosed in U.S. Pat. No. 4,668,642 include, among others, elemental boron, metaboric acid, orthoboric acid and organoboron compounds of the general formula BR.sub.3 " in which R" may be an alkyl, aryl or alkoxy substituent. In this case, the inorganic boron compounds are simply admixed with the silazane polymer and do not react until the material is pyrolyzed.
U.S. Pat. No. 4,482,689, L. A. Haluska, relates to a metallosilazane polymer containing boron which can be converted to a ceramic by pyrolyzing at temperatures above 750.degree. C., and is more specifically directed to a method for preparing a polymetallo(disilyl)silazane polymer that can be used in the formation of a ceramic material. The metallosilazane polymers containing boron are prepared by the reaction of a silazane with a mixture of chlorine-containing disilanes and a boron halide to produce boron-nitrogen bonds.
Seyferth and Plenio (J. Am. Ceram. Soc., 1990, 73(7), 2131-33) describe the preparation of borasilazane polymeric precursors for borosilicon nitride prepared by the reaction of a polysilazane, (CH.sub.3 SiHNH).sub.x, with borane-dimethylsulfide adduct to give a polymer with boron-nitrogen bonds.
Other boron-substituted, silicon-based preceramic polymers have been disclosed. These contain no nitrogen in the polymer backbone, and are generally polysilane- or polycarbosilane-based.
U.S. Pat. No. 4,283,376, Yajima et al., relates to production of silicon carbide fibers from a polycarbosilane containing boron in its side chain. The fibers are made infusible and fired in a vacuum or in an inert atmosphere. Yajima et al. thus teach a process for producing polycarbosilanes that, in part, contain siloxane bonds by the addition of polyborosiloxane.
U.S. Pat. No. 4,851,491, Riccitiello et al., discloses a polyorganoborosilane ceramic precursor polymer comprising a plurality of repeat units of boron bonded to silicon; the polyorganoborosilanes are useful in the preparation of SiC, SiB.sub.4, and B.sub.4 C.
U.S. Pat. No. 4,962,069, G. T. Burns et al., discloses a method to prepare a sintered silicon carbide body with a metal powder sintering aid and a preceramic polymer which forms a set amount of free carbon upon pyrolysis. In U.S. Pat. No. 4,962,069 the polysilazane precursors are used as a source of free carbon, and the sintering aid, i.e., boron or boron carbide, is added as a metal or ceramic powder.
U.S. Pat. No. 4,987,201, S. R. Riccitiello et al., discloses organoborosilicon preceramic polymers containing a silicon-carbon-boron backbone. These polymers are prepared by the hydroboration of monomeric vinyl- or acetylene-substituted alkylsilanes with aminoborane complexes or diborane. The silicon in the polymers disclosed in U.S. Pat. No. 4,987,201 may only be alkyl- or aryl-substituted and the boranes must contain only hydrogen substituents; also, the polymers described in U.S. Pat. No. 4,987,201 are boron-containing polycarbosilanes.
U.S. Pat. No. 4,041,117, S. Prochazka, discloses the preparation of a sintered silicon carbide body in which the boron, boron carbide or carbonaceous materials are used as sintering aids.
U.S. Pat. No. 4,108,929 S. Prochazka, discloses the preparation of a densified silicon carbide body by hot pressing a mixture of SiC with sintering aids, such as boron, boron carbide, or carbonaceous materials.
U.S. Pat. No. 4,962,069 G. T. Burns et al., discloses a method to prepare a sintered silicon carbide body with a metal powder sintering aid and a preceramic polymer which forms a set amount of free carbon upon pyrolysis. In U.S. Pat. No. 4,962,069, the polysilazane precursors are used as a source of free carbon, and the sintering aid, i.e., elemental boron or boron carbide is added as a powder.
Silicon carbide is a structural ceramic with good high temperature properties. Like other covalently bonded ceramics, such as silicon nitride, silicon carbide must be sintered with the addition of sintering aids. Sintering aids help to form a coherent bonded mass by mechanisms such as the formation of liquid phases or by enhancing solid state diffusion through the bulk or on the surface of the powdered particles. Unlike silicon nitride, which must be sintered via liquid phase sintering due to the decompositions which occur at about 1800.degree. C., silicon carbide is sintered via surface diffusion at 2100.degree. C. Silicon nitride, therefore, has poor high temperature properties due to the glassy intergranular phase. Silicon carbide, in contrast, maintains its properties to high temperature without degradation because there is no intergranular glassy phase which can soften. While it is possible to sinter silicon carbide via liquid phase sintering, the result would not be a useful high temperature ceramic.
Silicon carbide is currently sintered with boron and carbon, or aluminum and carbon additives. These additives increase the surface diffusion of silicon carbide. Prochazka, et al. (J. Am. Ceram. Soc. 1985, 68(9)479), found that SiC could be sintered to 97% of theoretical density at 2100.degree. C. with the addition of 0.5 wt % B and 1.5 wt % C. In this case, the boron must be well distributed throughout the powder and the carbon must be amorphous. These additives were introduced into the SiC powder by ball milling a mixture of the SiC powder with elemental boron or boron carbide for two hours in hexane. This traditional milling technique provides macroscopic mixing of the sintering aids throughout the bulk powder. Since the boron and carbon are added separately, however, regions that are rich in boron or carbon may be formed.